Polarographic electrodes have been found to be an invaluable tool for measuring the partial pressure of oxygen in blood (pO.sub.2) in extracorporeal loops such as those utilized during surgery. Numerous publications and patents have dealt with these electrodes, ever seeking designs which are reliable, rugged, and simple to manufacture. For a rather exhaustive treatment of the theory of oxygen electrode operation, and a presentation of most of the well known prior art designs, see "Medical and Biological Applications of Electrochemical Devices", Chapter 6, Edt. by J. Koryta, John Wiley & Sons Limited, 1980.
In its simplest form, a polarographic oxygen electrode consists of a noble metal cathode immersed in the solution to be monitored, and negatively biased with respect to a reference electrode (i.e. anode) which also is coupled to the solution. Oxygen content of the sample is measured amperometrically at the potential of the limiting current, at which point the oxygen vs. current characteristic defines a linear proportionality between oxygen content and electrical current.
Early work in the polarographic electrode field concerned protection of the metallic electrode system from surface contamination and poisoning by blood proteins. A watershed in this area occurred in a 1956 disclosure by L. C. Clark, Jr. (Trans. American Society Art. Int. Organs, Vol. 2, pp. 41-45, 1956), involving an entirely closed system wherein a cathode, an anode, and an electrolyte are physically separated from the solution to be measured by a hydrophobic membrane which is permeable only by the gas (e.g., oxygen) to be measured. See also U.S. Pat. No. 2,913,386 to Clark, issued Nov. 17, 1959, and U.S. Pat. No. 3,826,730 to Watanabe et al., issued July 30, 1974.
More recently, sensors have been developed which utilize a hydrophilic, rather than hydrophobic membrane to enclose the anode-cathode-electrolyte cell. Such sensors utilize membranes which are permeable not only by blood gases, but also by water and blood plasma electrolytes; they are not, however, permeable by proteins or other large molecules. Examples of hydrophilic materials known to be applicable in such cells are collodion, cellophane, polystyrene, "hydron", and the like. See, for example, the thesis of D. W. DeHaas, Rotterdam (1977), setting forth a sensor utilizing the membrane material commercially available under the trade name pHEMA (poly [2-hydroxyethyl methacrylate]).
Based on the foregoing precepts, other variants have been disclosed. For example, U.S. Pat. No. 3,700,579 to Clifton et al. discloses utilization of the water absorption characteristics of chlorinated fluorocarbon materials to employ them as membrane materials. Similar thinking has led to the use of tortuous pore membranes prepared from mixed esters of cellulose.
Each of the foregoing approaches to the design of polarographic sensors involves rather severe drawbacks. Moreover, though the physical and dynamic mechanisms are different for each, substantially all have problems relating to electrical drift and instability, poor reusability characteristics, poor mechanical strength, difficulties of calibration, and excessive complexity of construction. pHEMA membranes are subject to degradation by hydroxyl ions generated during the electro reduction of oxygen in the sensor. Membranes based on chlorinated fluorocarbon materials (e.g. polytetrafluoroethylene) must be activated by forcing aqueous solution into the polymeric matrix by steam sterilization. Even so, such sensors are generally sensitive to osmotic effects. Cellulosic membranes stretch when fully hydrated, leading to changes in electrolyte dimension and consequent sensor instability. Furthermore, glycerin, which is added to the membrane during manufacture to achieve requisite flexibility, leaches out of the membrane during first use. Redrying of the previously hydrated membrane yields a very brittle material, lacking not only mechanical strength, but in any event making it substantially impossible to precalibrate.
It is a primary object of the present invention to provide a polarographic oxygen sensor design and construction which substantially improves upon the previously characterized prior art approaches, substantially eliminating or at least vastly decreasing prior art problems of structural complexity, electrical instability, membrane strength, and the like. More specific objects of the present invention include providing a sensor construction and membrane selection characterized by rapid hydration, resistance to chemical degradation or alteration during the sensing process, physical and mechanical stability, compatibility with blood, precalibration facility, and rapidity of response.